Process for the preparation of precipitated silica, new precipitated silicas containing zinc and their use for the reinforcement of elastomers

ABSTRACT

The invention relates to a new process for the preparation of precipitated silica which has a good dispersibility and very satisfactory reinforcing properties. It also relates to new precipitated silicas which are in the form of powder, of substantially spherical beads or of granules, these silicas being characterized by the fact that they have a CTAB specific surface of between 90 and 250 m 2  /g, a DOP oil uptake lower than 300 ml/100 g, a pore distribution such that the pore volume consisting of the pores whose diameter is between 175 and 275 Å represents less than 50% of the pore volume consisting of the pores of diameters smaller than or equal to 400 Å, a zinc content of between 1 and 5% by weight and by the fact that the number N of molecules of stearic acid consumed per nm 2  of silica surface, when stearic acid is reacted with the said silica in xylene for 2 hours at 120° C., is at least 1. The invention also relates to the use of the said silicas as reinforcing fillers for elastomers especially for improving their rheological properties.

This application is a 371 of international application number PCT/FR96/00461, filed Mar. 28,1996.

The present invention relates to a new process for the preparation of precipitated silica, to precipitated silicas which are in particular in the form of powder, of substantially spherical beads or of granules, and to their application as a reinforcing filler for elastomers.

It is known that precipitated silica has been employed for a long time as a white reinforcing filler in elastomers.

However, like any reinforcing filler, it is appropriate that it should be capable of, on the one hand, being handled and above all, on the other hand, of being easily incorporated into the mixtures.

It is known in general that, to obtain the optimum reinforcing properties conferred by a filler, it is appropriate that the latter should be present in the elastomer matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. However, such conditions can be achieved only insofar as, on the one hand, the filler has a very good ability to be incorporated into the matrix during mixing with the elastomer (incorporability of the filler) and to disintegrate or to deagglomerate into the form of a very fine powder (disintegration of the filler) and as, on the other hand, the powder resulting from the abovementioned disintegration process can itself, in its turn be perfectly and homogeneously dispersed in the elastomer (dispersion of the powder).

Moreover, for reasons of mutual affinities, silica particles have an unfortunate tendency, in the elastomer matrix, to agglomerate with each other. These silica/silica interactions have a detrimental consequence of limiting the reinforcing properties to a level that is substantially lower than that which it would be theoretically possible to expect if all the silica/elastomer interactions capable of being created during the mixing operation were actually obtained (this theoretical number of silica/elastomer interactions being, as is well known, directly proportional to the external surface of the silica employed).

Furthermore, in the raw state, such silica/silica interactions tend to increase the stiffness and the consistency of the mixtures, thus making them more difficult to process.

The problem arises of having available fillers which, while being capable of being relatively large in size, improve the rheological properties of elastomers and advantageously have a good dispersibility in elastomers.

The aim of the present invention is to overcome the abovementioned disadvantages and to solve the abovementioned problem.

More precisely, its aim is especially to propose a new process for the preparation of precipitated silica which, advantageously, has a good dispersibility (and disintegratability) and very satisfactory reinforcing properties, in particular which, when employed as a reinforcing filler for elastomers, imparts excellent Theological properties to the latter while providing them with good mechanical properties.

The invention also relates to precipitated silicas which, preferably, are in the form of powder, of substantially spherical beads or, optionally, of granules, and which, while being of relatively large size, have very satisfactory reinforcing properties and, in an advantageous manner, very good dispersibility (and disintegratability).

It relates, finally, to the use of the said precipitated silicas as reinforcing fillers for elastomers.

In the description which follows, the BET specific surface is determined according to the Brunauer-Emmett-Teller method described in the Journal of the American Chemical Society, Vol. 60, page 309, February 1938 and corresponding to NFT standard 45007 (November 1987).

The CTAB specific surface is the outer surface determined according to NFT standard 45007 (November 1987) (5.12).

The DOP oil uptake is determined according to NFT standard 30-022 (March 1953) by using dioctyl phthalate.

The packing density (PD) is measured according to NFT standard 030100.

The pH is measured according to ISO standard 787/9 (pH of a suspension at a concentration of 5% in water).

Finally, it is specified that the given pore volumes are measured by mercury porosimetry, the pore diameters being calculated from the Washburn relationship with an angle of contact theta equal to 130° and a surface tension gamma equal to 484 dynes/cm (MICROMERITICS 9300 POROSIMETER® ).

The dispersibility and the disintegratability of the silica according to the invention can be quantified by means of a specific disintegration test.

The disintegration test is carried out according to the following procedure:

the cohesion of the agglomerates is assessed by a particle size measurement (using laser scattering), performed on a silica suspension previously disintegrated by ultrasonic treatment; the disintegratability of the silica is thus measured (rupture of objects from 0.1 to a few tens of microns). The disintegration under ultrasound is performed with the aid of a VIBRACELL BIOBLOCK (600 W)® sonic transducer equipped with a probe 19 mm in diameter. The particle size measurement is performed by laser scattering on a Sympatec particle size analyser.

2 grams of silica are measured out into a specimen tube (height: 6 cm and diameter 4 cm) and are made up to 50 grams by adding demineralized water; an aqueous suspension containing 4% of silica is thus produced, which is homogenized for 2 minutes by magnetic stirring. The disintegration under ultrasound is next performed as follows: with the probe immersed to a depth of 4 cm, the power is adjusted so as to obtain a needle deflection on the power dial indicating 2% (which corresponds to an energy of 120 watts/cm² dissipated by the end of the probe). The disintegration is performed for 420 seconds. The particle size measurement is then carried out after a known volume (expressed in ml) of the homogenized suspension has been introduced into the cell of the particle size analyser.

The value of the median diameter .O slashed.₅₀ which is obtained is proportionally smaller the higher the disintegratability of the silica. The ratio (10×volume of dispersion introduced (in ml))/optical detected by the particle size analyser (this optical density is of the order of 20) is also determined. This ratio is an indication of the proportion of fines, that is to say of the content of particles smaller than 0.1 μm, which are not detected by the particle size analyser. This ratio, called the ultrasonic disintegration factor (F_(D)) is proportionally higher the higher the disintegratability of the silica.

One of the subjects of the invention is a process for the preparation of precipitated silica of the type including the reaction of a silicate with an acidifying agent, whereby a suspension of precipitated silica is obtained, followed by the separation and the drying of this suspension, in which the precipitation is carried out in the following manner:

(i) an initial base stock comprising a silicate of alkali metal M, and an electrolyte is formed, the silicate concentration (expressed as SiO₂) in the said initial base stock being lower than 20 g/l,

(ii) the acidifying agent is added to the said base stock until at least 5% of the quantity of M₂ O present in the said base stock is neutralized,

(iii) acidifying agent and a silicate of alkali metal M are added simultaneously to the reaction mixture, such that the ratio of the quantity of silicate added (expressed as SiO₂)/the quantity of silicate present in the initial base stock (expressed as SiO ₂), called the degree of consolidation, is greater than 4 and at most 100,

characterized in that the said process includes one of the following two operations (a) or (b):

(a) at least one zinc compound and then a basic agent are added to the reaction mixture after stage (iii) and, when the said separation comprises a filtration and a disintegration of the cake originating from this filtration, the said disintegration is preferably performed in the presence of at least one aluminium compound,

(b) a silicate and at least one zinc compound are added simultaneously to the reaction mixture after stage (iii) and, when the said separation comprises a filtration and a disintegration of the cake originating from this filtration, the disintegration is preferably performed in the presence of at least one aluminium compound.

It has thus been found that the introduction of zinc--this being according to a particular method--combined with a low silicate concentration (expressed as SiO₂) in the initial base stock and with an appropriate degree of consolidation during the simultaneous addition stage constitutes an important condition for imparting their good properties to the products obtained, especially very satisfactory reinforcing properties (in particular in respect of the rheology of the elastomers).

It should be noted, in general, that the process concerned is a process for the synthesis of precipitated silica, that is to say that an acidifying agent is reacted with a silicate in very special conditions.

The choice of the acidifying agent and of the silicate is made in a manner which is well known per se.

It may be recalled that the acidifying agent generally employed is a strong inorganic acid such as sulphuric acid, nitric acid or hydrochloric acid, or an organic acid such as acetic acid, formic acid or carbonic acid.

The acidifying agent may be dilute or concentrated; its normality may be between 0.4 and 36N, for example between 0.6 and 1.5N.

In particular, in the case where the acidifying agent is sulphuric acid, its concentration may be between 40 and 180 g/l , for example between 60 and 130 g/l.

It is possible, furthermore, to employ as a silicate any common form of silicates such as metasilicates, disilicates and advantageously an alkali metal silicate, especially sodium or potassium silicate.

The silicate may exhibit a concentration, expressed as silica, of between 40 and 330 g/l, for example between 60 and 300 g/l, in particular between 60 and 250 g/l.

In general, sulphuric acid is employed as the acidifying agent, and sodium silicate as the silicate.

In the case where sodium silicate is employed, the latter generally exhibits an SiO₂ /Na₂ O weight ratio of between 2 and 4, for example between 3.0 and 3.7.

Insofar as the process of preparation of the invention is more particularly concerned, the precipitation is done in a specific manner according to the following stages.

First of all a base stock is formed which includes some silicate (stage (i)). The quantity of silicate present in the initial base stock advantageously represents only a part of the total quantity of silicate introduced into the reaction.

According to a characteristic of the process of preparation according to the invention, the silicate concentration in the initial base stock is (higher than 0 g/l and) lower than 20 g of SiO₂ per liter. This concentration may be at most 11 g/l and, optionally, at most 8 g/l.

In particular when the separation performed subsequently during the process according to the invention includes a filtration performed by means of a filter press (and more particularly in the case where it is desired to prepare silicas which have a CTAB specific surface of at least 140 m² /g), this concentration may be at least 8 g/l, in particular between 10 and 15 g/l, for example between 11 and 15 g/l; the drying applied later in the process according to the invention may be performed by spraying by means of a multinozzle sprayer.

The base stock may include an electrolyte. Nevertheless, preferably, no electrolyte is employed in the course of the process of preparation according to the invention; in particular, the initial base stock preferably does not include any electrolyte.

The term electrolyte is intended to be understood here in its normal accepted meaning, that is to say that it means any ionic or molecular substance which, when it is in solution, decomposes or dissociates to form ions or charged particles. A salt from the group of the alkali metal and alkaline-earth metal salts may be mentioned as an electrolyte, especially the salt of the metal of the starting silicate and of the acidifying agent, for example sodium sulphate in the case of the reaction of a sodium silicate with sulphuric acid.

The second stage consists in adding the acidifying agent to the base stock of composition described above (stage (ii)).

Thus, in this second stage, the acidifying agent is added to the said initial base stock until at least 5%, preferably at least 50%, of the M₂ O quantity present in the said initial base stock is neutralized.

In this second stage the acidifying agent is preferably added to the initial base stock until 50 to 99% of the quantity of M₂ present in the said initial base stock is neutralized.

Once the desired value of neutralized M₂ O quantity is reached, then a simultaneous addition (stage (iii)) of acidifying agent and of a quantity of silicate of alkali metal M is undertaken, such that the degree of consolidation, that is to say the ratio of the quantity of silicate added (expressed as SiO₂)/the quantity of silicate present in the initial base stock (expressed as SiO₂) is higher than 4 and at most 100.

According to an alternative form of the process of the invention, this simultaneous addition of acidifying agent and of a quantity of silicate of alkali metal M is undertaken such that the degree of consolidation is more particularly between 12 and 100, preferably between 12 and 50, especially between 13 and 40.

According to another alternative form of the process of the invention this simultaneous addition of acidifying agent and of a quantity of silicate of alkali metal M is undertaken such that the degree of consolidation is rather higher than 4 and lower than 12, preferably between 5 and 11.5, especially between 7.5 and 11. This alternative form is, in general, used when the silicate concentration in the initial base stock is at least 8 g/l, in particular between 10 and 15 g/l, for example between 11 and 15 g/l.

Throughout the stage (iii) the quantity of acidifying agent which is added is preferably such that 80 to 99%, for example 85 to 97%, of the quantity of M₂ O which is added is neutralized.

In stage (iii) it is possible to undertake the simultaneous addition of acidifying agent and of silicate at a first pH plateau of the reaction mixture, pH₁ , and then at a second pH plateau of the reaction mixture, pH₂, such that 7<pH₂ <pH₁ <9.

According to an essential characteristic of the process of preparation according to the invention, the latter includes one of the two operations, (a) or (b) mentioned above, that is to say:

(a) at least one zinc compound and then a basic agent are added, and, when the separation used in the process comprises a filtration and a disintegration of the cake originating from this filtration, the said disintegration is preferably performed in the presence of at least one aluminium compound, or

(b) a silicate and at least one zinc compound are added simultaneously, after stage (iii), to the reaction mixture and, when the separation used in the process comprises a filtration, the disintegration of the cake originating from this filtration is preferably performed in the presence of at least one aluminium compound.

In a first alternative form of the process of preparation according to the invention (that is to say when the latter includes the operation (a)), the following successive stages are performed advantageously after having carried out the precipitation according to the stages (i), (ii) and (iii) described above:

(iv) at least one zinc compound is added to the reaction mixture (that is to say to the reaction suspension or slurry obtained),

(v) a basic agent is added to the reaction mixture preferably until a pH value of the reaction mixture of between 7.4 and 10, in particular between 7.8 and 9, is obtained,

(vi) acidifying agent is added to the reaction mixture, preferably until a pH value of the reaction mixture of at least 7, in particular between 7 and 8.5, for example between 7 and 8, is obtained.

After the simultaneous addition of stage (iii) it may then be advantageous to perform a maturing of the reaction mixture, it being possible for this maturing to last, for example, from 1 to 60 minutes, in particular from 3 to 30 minutes.

In this first alternative form it is desirable, between stage (iii) and stage (iv), and especially before the said optional maturing, to add an additional quantity of acidifying agent to the reaction mixture. This addition is generally done until a pH value of the reaction mixture of between 3 and 6.5, in particular between 4 and 6, is obtained.

The acidifying agent employed during this addition is generally identical with that employed during stages (ii), (iii) and (vi) of the first alternative form of the process of preparation according to the invention.

A maturing of the reaction mixture is usually performed between stage (v) and (vi), for example for 2 to 60 minutes, in particular for 5 to 45 minutes.

Similarly, a maturing of the reaction mixture is in most cases performed after stage (vi), for example for 2 to 60 minutes, in particular for 5 to 30 minutes.

The basic agent employed during stage (iv) may be a solution of aqueous ammonia or, preferably, a solution of sodium hydroxide (or soda).

In a second alternative form of the process of preparation according to the invention (that is to say when the latter includes the operation (b)), a stage (iv) is performed after the stages (i), (ii) and (iii) described previously, which consists in adding a silicate and at least one zinc compound simultaneously to the reaction mixture.

After the simultaneous addition of stage (iv) it may then be advantageous to perform a maturing of the reaction mixture, it being possible for this maturing to last, for example, from 2 to 60 minutes, in particular from 5 to 30 minutes.

In this second alternative form it is desirable, after stage (iv), and especially after this optional maturing, to add an additional quantity of acidifying agent to the reaction mixture. This addition is generally done until a pH value of the reaction mixture of at least 7, in particular between 7 and 8.5, for example between 7 and 8, is obtained.

The acidifying agent employed during this addition is generally identical with that employed during stages (ii) and (iii) of the second alternative form of the process of preparation according to the invention.

A maturing of the reaction mixture is usually performed after this addition of acidifying agent, for example for 1 to 60 minutes, in particular for 3 to 30 minutes.

The zinc compound employed in the process of preparation according to the invention is generally an organic or inorganic zinc salt.

By way of examples of an organic salt there may be mentioned especially the salts of carboxylic or polycarboxylic acids, like the salts of acetic, citric, tartaric or oxalic acid.

By way of examples of an inorganic salt there may be mentioned especially halides and oxyhalides (like chlorides and oxychlorides), nitrates, phosphates, sulphates and oxysulphates.

In practice, the zinc compound may be employed in the form of a solution, generally aqueous.

A zinc sulphate is preferably employed as zinc compound.

The temperature of the reaction mixture is generally between 60° and 98° C.

According to an alternative form of the invention the reaction is performed at a constant temperature of between 70° and 96° C.

According to another alternative form of the invention the temperature at the end of the reaction is higher than the temperature at the beginning of reaction; the temperature at the beginning of the reaction is thus maintained preferably between 70° and 96° C. and the temperature is then raised over a few minutes, preferably up to a value of between 75° and 98° C., which value it is maintained until the end of the reaction; the operations (a) or (b) are thus usually performed at this constant temperature value.

At the outcome of the stages which have just been described a silica slurry is obtained which is then separated (liquid-solid separation).

In the first alternative form of the process of the preparation according to the invention (that is to say when the latter includes the operation (a)), this separation comprises, in general, a filtration (followed by washing if necessary) and a disintegration, the said disintegration being performed in the presence of at least one zinc compound and, preferably, in the presence of an acidifying agent as described above (in this latter case the zinc compound and the acidifying agent are advantageously added simultaneously).

The disintegration operation, which may be carried out, for example, by passing the filter cake through a mill of the colloid or bead type, makes it possible in particular to lower the viscosity of the suspension to be subsequently dried.

In the second alternative form of the process of preparation according to the invention (that is to say when the latter includes the operation (b)), the separation also comprises, in general, a filtration (followed by washing if necessary) and a disintegration, the said disintegration being preferably performed in the presence of at least one aluminium compound and, in general, in the presence of an acidifying agent as described above (in this latter case the aluminium compound and the acidifying agent are advantageously added simultaneously).

The aluminium compound generally consists of an alkali metal, especially potassium, or, very preferably, sodium, aluminate.

The quantity of the zinc compound employed in the process of preparation according to the invention is preferably such that the precipitated silica prepared contains more between 1 and 5%, in particular between 1.5 and 4%, for example between 1.5 and 2.5%, by weight of zinc.

The separation used in the process of preparation according to the invention usually includes a filtration performed by means of any suitable method, for example by means of a belt filter, a rotary vacuum filter or, preferably, a filter press.

The suspension of precipitated silica thus recovered (filter cake) is then dried.

This drying may be done according to any method that is known per se.

The drying is preferably done by spraying.

Any suitable type of sprayer may be employed for this purpose, especially a turbine, nozzle, liquid-pressure or two-fluid sprayer.

The drying is, for example, performed by spraying by means of a multinozzle sprayer especially when the concentration of silicate in the initial base stock is at least 8 g/l (and lower than 20 g/l), in particular between 10 and 15 g/l (and more particularly in the case when it is desired to prepare silicas which have a CTAB specific surface of at least 140 m² /g).

According to one embodiment of the invention, the suspension to be dried has a solids content higher than 15% by weight, preferably higher than 17% by weight and, for example, higher than 20% by weight. The drying is then preferably performed by means of a multinozzle sprayer.

The precipitated silica capable of being obtained according to this embodiment of the invention and preferably by using a filter press is advantageously in the form of substantially spherical beads, preferably of a mean size of at least 80 μm.

It should be noted that dry material for example silica in pulverulent form may be also added to the filter cake after the filtration, at a subsequent stage of the process.

At the end of the drying, a stage of milling may be undertaken on the product recovered, especially on the product obtained by drying a suspension which has a solids content higher than 15% by weight. The precipitated silica which is then obtainable is generally in the form of a powder, preferably with a mean size of at least 15 μm, in particular between 15 and 60 μm, for example between 20 and 45 μm.

The milled products with the desired particle size can be separated from any nonconforming products by means, for example, of vibrating screens which have appropriate mesh sizes, and the nonconforming products thus recovered can be returned to the milling.

Similarly, according to another embodiment of the invention, the suspension to be dried has a solids content of at most 15% by weight. The drying is then generally performed by means of a turbine sprayer. The precipitated silica which is then obtainable according to this embodiment of the invention and preferably by using a rotary vacuum filter is generally in the form of a powder, preferably with a mean size of at least 15 μm, in particular between 30 and 150 μm, for example between 45 and 120 μm.

Finally, the product which has been dried (especially from a suspension which has a solids content of at most 15% by weight) or milled can, according to another embodiment of the invention, be subjected to an agglomeration stage.

Agglomeration is here intended to mean any process which enables finely divided objects to be bonded together in order to bring them into the form of objects of larger size and which are mechanically stronger.

These processes are especially direct compression, wet-route granulation (that is to say with the use of a binder such as water, silica slurry, etc.), extrusion and, preferably, dry compacting.

When this last technique is used it may be found advantageous, before starting the compacting, to deaerate the pulverulent products (an operation which is also called predensifying or degassing), so as to remove the air included therein and to ensure a more uniform compacting.

The precipitated silica which can be obtained according to this embodiment of the invention is advantageously in the form of granules, preferably at least 1 mm in size, in particular between 1 and 10 mm.

At the end of the agglomeration stage the products may be classified to a desired size, for example by screening, and then packaged for their future use.

The powders, as well as the beads, of precipitated silica which are obtained by the process according to the invention thus offer the advantage, among others, of providing access to granules such as those mentioned above, in a simple, efficient and economical manner, especially by conventional forming operations, such as, for example, granulation or compacting, without the latter resulting in degradation capable of masking, or even annihilating, the good intrinsic properties associated with these powders or these beads, as may be the case in the prior art when using conventional powders.

Other subjects of the invention consist of new precipitated silicas which have, in an advantageous manner, a good dispersibility (and disintegratability) and very satisfactory reinforcing properties, in particular which, when employed as a reinforcing filler for elastomers, impart very good rheological properties to the latter while providing them with very satisfactory mechanical properties.

Thus, a new precipitated silica is now proposed, according to the invention, characterized in that it has:

a CTAB specific surface of between 90 and 250 m² /g, for example between 120 and 230 m² /g,

a DOP oil uptake lower than 300 ml/100 g, preferably between 200 and 295 ml/100 g,

a pore distribution such that the pore volume consisting of the pores whose diameter is between 175 and 275 Å represents less than 50% of the pore volume consisting of the pores of diameters which are smaller than or equal to 400 Å,

a zinc of between 1 and 5% by weight, preferably between 1.5 and 4% by weight, and in that the number N of molecules of stearic acid consumed per nm² of silica surface, when stearic acid is reacted with the said silica in xylene for 2 hours at 120° C., is at least 1, preferably at least 1.2, in particular at least 1.5.

The silica according to the invention preferably has a zinc content of between 1.5 and 4% by weight; this content may be especially between 1.5 and 2.5% by weight.

One of the essential characteristics of the precipitated silica according to the invention is its consumption, in a model medium (xylene), of an ingredient of rubber vulcanization (stearic acid).

The Applicant Company has thus found that the precipitated silicas exhibiting a particular number N, in combination with the other characteristics mentioned in the present description, made it possible in particular to impart very good rheological properties to the elastomers while providing them with satisfactory mechanical properties.

To determine this characteristic (number N), stearic acid is reacted in the presence of silica in xylene for 2 hours at 120° C. The quantity of stearic acid remaining in the xylene after reaction is then determined by infrared (IR) spectrometry; the quantity of stearic acid which has been consumed by the silica can then be deduced and, hence, the number N of molecules of stearic acid consumed per nm² of the silica surface.

The operating method employed for determining this characteristic is described more precisely below.

60.2 g (that is 70 ml) of xylene are added into a round bottom flask containing 3.17 g of stearic acid. The flask is stoppered and is then magnetically stirred for a few minutes.

12.04 g of silica are then added.

The flask is placed in an oil bath at 1200° C. under reflux (a condenser being fitted to it). The flask is then magnetically stirred for 105 min. The stirring is then stopped and the flask is left in the oil bath for another 15 min. The total duration of the reaction at 120° C. is therefore 2 hours.

The condenser is removed and the flask is taken out of the oil bath.

The contents of the flask are filtered on a microfiltration system (Millipore unit with Durapore membrane filters made of polyvinylidene fluoride (pore size: 0.45 μm)).

10 g of the filtrate obtained are then diluted in 10 g of xylene; a solution S is obtained.

In parallel, standard solutions of stearic acid in xylene are prepared (which have a stearic acid content lower than 2 mass %) and the IR spectra (from 400 to 4000 cm⁻¹) of each of them are produced. The characteristic peak of stearic acid is situated at 1710 cm⁻¹. The intensity of this peak associated with the stearic acid content of the solution makes it possible to plot the straight line of the stearic acid content of the solution as a function of the IR absorbance at 1710 cm⁻¹., the equation of the calibration straight line is obtained by linear regression.

Similarly, the IR spectrum of the solution S is produced. The value of the characteristic peak of stearic acid, referred to the equation of the calibration straight line, allows the content of stearic acid present in the solution S to be determined; by taking into account the mass of xylene added during the dilution, the stearic acid content of the filtrate from the reaction is obtained. The content and therefore the quantity of stearic acid consumed by the silica during the reaction are deduced from the initial stearic acid content and from the stearic acid content after reaction (the latter being the stearic acid content of the filtrate). The number N of molecules of stearic acid which are consumed per nm² of the silica surface is then determined.

The zinc present in the precipitated silica according to the invention is preferably not in crystalline form, but is rather present in amorphous form (this can be determined by X-ray diffraction).

Another characteristic of the silica according to the invention lies in the distribution, or spread, of the pore volume and especially in the distribution of the pore volume which is produced by the pores of diameters smaller than or equal to 400 Å. This latter volume corresponds to the useful pore volume of the fillers which are employed in the reinforcement of elastomers. Analysis of the porograms shows that the silica according to the invention then has a pore distribution such that the pore volume consisting of the pores whose diameter is between 175 and 275 Å represents less than 50%, for example less than 40%, of the pore volume consisting of the pores of diameters smaller than or equal to 400 Å.

According to a first (preferred) particular embodiment of the invention, the precipitated silica has:

a CTAB specific surface of between 90 and 185 m² /g,

a median diameter (.O slashed.₅₀), after disintegration with ultrasound, smaller than 6 μm, preferably smaller than 5 μm.

It then generally has a BET specific surface of between 90 and 230 m² /g, in particular between 100 and 190 m² /g, for example between 120 and 190 m² /g.

According to an alternative form of this embodiment of the invention the precipitated silica has:

a CTAB specific surface of between 90 and 140 m² /g, for example between 100 and 135 m² /g, for example between 120 and 135 m² /g,

a median diameter (.O slashed.₅₀), after disintegration with ultrasound, smaller than 4.5 μm, in particular smaller than 4 μm, for example smaller than 3.8 μm.

According to another alternative form of this embodiment of the invention the precipitated silica has:

a CTAB specific surface of between 140 and 185 m² /g,

an ultrasonic disintegration factor (F_(D)) higher than 5.5 ml, in particular higher than 11 ml.

According to a second particular embodiment of the invention the precipitated silica has:

a CTAB specific surface higher than 185 m² m/g and lower than 220 m² /g,

a median diameter (.O slashed.₅₀ ), after disintegration with ultrasound, smaller than 8.5 μm, preferably smaller than 7 μm.

It then generally has a BET specific surface of between 190 and 280 m² /g, especially between 190 and 250 m² /g.

The ultrasonic disintegration factor (F_(D)) of the precipitated silica according to this particular embodiment of the invention may be higher than 5.5 ml.

According to an alternative form of the invention the silica has a BET specific surface/CTAB specific surface ratio of between 1.0 and 1.2, that is to say that it preferably exhibits very low microporosity.

According to another alternative form of the invention the silica has a BET specific surface/CTAB specific surface ratio higher than 1.2, for example of between 1.21 and 1.4, that is to say that it exhibits a relatively high microporosity.

The pH of the silica according to the invention is generally between 8.0 and 9.0, for example between 8.3 and 8.9.

The silicas according to the invention may be in the form of powders, of substantially spherical beads or, optionally, of granules, and are characterized particularly by the fact that, while being relatively large in size, they have very satisfactory reinforcing properties and, preferably, a very good dispersibility and disintegratability.

The silica powders according to the invention preferably have a mean size of at least 15 μm; the latter is, for example, between 15 and 60 μm (especially between 20 and 45 μm) or between 30 and 150 μm (especially between 45 and 120 μm).

They have, preferably, a DOP oil uptake of between 240 and 290 ml/100 g.

The packing density (PD) of the said powders is generally at least 0.17 and, for example, between 0.2 and 0.3.

The said powders generally have a total pore volume of at least 2.5 cm³ /g and, more particularly, of between 3 and 5 cm³ /g.

They make it possible in particular to obtain a very good compromise between processing and mechanical properties in the vulcanized state.

They also constitute preferred precursors for the synthesis of granulates as described later.

The substantially spherical beads according to the invention preferably have a mean size of at least 80 μm.

According to certain alternative forms of the invention, this mean bead size is at least 100 μm, for example at least 150 μm; it is generally at most 300 μm and preferably lies between 100 and 270 μm. This mean size is determined according to NF standard ×11507 (December 1970) by dry screening and determination of the diameter corresponding to a cumulative oversize of 50%.

They preferably have a DOP oil uptake of between 240 and 290 ml/100 g.

The packing density (PD) of the said beads (or prills) is generally at least 0.17 and, for example, between 0.2 and 0.34.

They usually have a total pore volume of at least 2.5 cm³ /g and, more particularly, of between 3 and 5 cm³ /g.

As indicated above, such a silica in the form of substantially spherical beads which are advantageously solid, homogeneous and low in dust and have good pourability, has a good disintegratability and dispersibility. In addition, it exhibits good reinforcing properties. Such a silica also constitutes a preferred precursor for the synthesis of the powders and granules according to the invention.

Such a silica in the form of substantially spherical beads constitutes a highly advantageous alternative form of the invention.

The dimensions of the granules according to the invention are preferably at least 1 mm, in particular between 1 and 10 mm, along the axis of their largest dimension (length).

They preferably have a DOP oil uptake of between 200 and 260 ml/100 g.

The said granules may be of the most diverse shape. The shapes which may be especially mentioned by way of example are the spherical, cylindrical, parallelepipedal, tablet, flake, pellet and extrudate of circular or polylobar section.

The packing density (PD) of the said granules is generally at least 0.27 and may range up to 0.37.

They generally have a total pore volume of at least 1 cm³ /g and, more particularly, between 1.5 and 2 cm³ /g.

The silicas according to the invention, especially in the form of powder or of substantially spherical beads or granules are preferably prepared according to one of the appropriate alternative forms of the process of preparation in accordance with the invention and described above.

The silicas according to the invention or prepared by the process according to the invention find a particularly advantageous application in the reinforcement of natural or synthetic elastomers. They impart excellent rheological properties to these elastomers while providing them with good mechanical properties and, in general, good resistance to abrasion. In addition, these elastomers are preferably less liable to heating.

The invention therefore also relates to the use of these silicas for improving the rheological properties of elastomers.

The following examples illustrate the invention without, however, limiting its scope.

EXAMPLE 1

The following were introduced into a stainless steel reactor provided with a stirring system using propellers and with heating using a jacket:

733 liter of water, and

46.5 liter of a solution of sodium silicate (SiO₂ /Na₂ O weight ratio of 3.4) with a concentration expressed as silica of 235 g/l.

The concentration of silicate expressed as SiO₂ in the initial base stock is thus 14 g/l. The temperature of the solution was then raised to 80° C. while being kept stirred. The entire reaction was carried out at 80° C. and with stirring. Dilute sulphuric acid with a density of 1.050 at 20° C. was then introduced at a rate of 5.4 l/min for a period of 9 minutes; following this addition, the neutralization ratio in the base stock was 78%, i.e., 78% of the quantity of Na₂ O present in the initial base stock had been neutralized.

Simultaneous introduction of a sodium silicate solution of the type described above at a rate of 4.3 l/min and of dilute sulphuric acid also of the type described above and at a rate which was regulated sa as to maintain a pH:

of 8.5±0.1 for the first 55 minutes, then

of 7.8±0.1 for the final 35 minutes,

in reaction medium, was then effected over 90 minutes.

During this simultaneous addition step, the instantaneous neutralisation ratio was 94%, i.e., 94% of the quantity of Na₂ O added (per min) was neutralized.

The consolidation ratio following simultaneous addition was 8.3.

After this simultaneous addition an aqueous solution containing 85 g/l of zinc sulphate is next introduced in the reaction medium for 12 minutes at a flow rate of 9.3 l/min. At the end of this addition an aqueous solution containing 180 g/l of sodium hydroxide is introduced into the reaction medium until the pH of the reaction mixture is equal to 8.9.

The introduction of sodium hydroxide is then stopped and the reaction medium is kept stirred for 10 minutes.

Sulphuric acid of the type described above is then introduced until the pH of the reaction medium is equal to 7.1.

The introduction of acid is then stopped and maturing of the reaction medium is undertaken for 5 minutes at temperature of 80° C.

The total reaction period is 148 minutes.

A slurry or suspension of precipitated silica is thus obtained, which is next filtered and washed by means of a filter press.

The cake obtained is next fluidized by mechanical and chemical action (simultaneous addition of sulphuric acid and of a quantity of sodium aluminate corresponding to an Al/SiO₂ weight ratio of 0.30%). After this disintegration operation, the resulting slurry, with a pH equal to 8.4 and a loss on ignition equal to 78.0% (and therefore a solids content of 22.0% by weight), is sprayed by means of a nozzle sprayer.

The characteristics of the silica P1 obtained in the form of substantially spherical beads are as follows:

    ______________________________________                                         CTAB speciflc surface   145 m.sup.2 /g                                         BET specific surface    175 m.sup.2 /g                                         DOP oil uptake          275 ml/100 g                                           Zinc weight content     1.80%                                                  Pore volume V1 represented                                                                             0.95 cm.sup.3 /g                                       by the pores of d ≦ 400 Å                                           Pore volume V2 represented                                                                             0.40 cm.sup.3 /g                                       by the pores 175 Å ≦ d ≦ 275 Å                           V2/V1 ratio             42%                                                    pH                      8.5                                                    Mean particle size      210 μm                                              ______________________________________                                    

The number N of stearic acid molecules consumed per nm² of silica surface, when stearic acid is reacted with the said silica P1 in xylene for 2 hours at 120° C. (in accordance with the operating procedure outlined in the description), is equal to 1.4.

The silica P1 is subjected to the disintegration test as defined above in the description.

After disintegration with ultrasound it has a median diameter (.O slashed.₅₀ ) of 2.7 μm and an ultrasonic disintegration factor (F_(D)) Of 16 ml.

EXAMPLE 2

The following were introduced into a stainless steel reactor provided with a stirring system using propellers and with heating using a jacket:

626 liters of water, and

36 liters of a solution of sodium silicate (SiO₂ /Na₂ O weight ratio of 3.4) with a concentration expressed as silica of 130 g/l.

The concentration of silicate expressed as SiO₂ in the initial base stock is thus 7.1 g/l. The temperature of the solution was then raised to 95° C. while being kept stirred. The entire reaction was carried out at 95° C. and with stirring. A sulphuric acid with a concentration of 80 g/l was then introduced at a rate of 5.4 l/min for a period of 3 minutes and 20 seconds; following this addition, the neutralisation ratio in the base stock was 67%, i.e., 67% of the quantity of Na₂ O present in the initial base stock had been neutralized.

Simultaneous introduction in the reaction medium of:

a sulphuric acid solution of the type described above at a rate of 5.4 l/min, and

a sodium silicate solution of the type described above at a rate of 9.2 l/min

was then effected over 70 minutes.

During this simultaneous addition step, the instantaneous neutralisation ratio was 79%, i.e., 79% of the quantity of Na₂ O added (per min) was neutralized.

The consolidation ratio following simultaneous addition was 17.9.

After this simultaneous addition an aqueous solution containing 85 g/l of zinc sulphate is next introduced in the reaction medium for 12 minutes at a flow rate of 9.3 l/min. At the end of this addition an aqueous solution containing 180 g/l of sodium hydroxide is introduced into the reaction medium until the pH of the reaction mixture is equal to 8.9.

The introduction of sodium hydroxide is then stopped and the reaction medium is kept stirred for 10 minutes.

Sulphuric acid of the type described above is then introduced until the pH of the reaction medium is equal to 7.1.

The introduction of acid is then stopped and maturing of the reaction medium is undertaken for 5 minutes at temperature of 95° C.

The total reaction period is 127 minutes.

A slurry or suspension of precipitated silica is thus obtained, which is next filtered and washed by means of a filter press.

The cake obtained is next fluidized by mechanical and chemical action (simultaneous addition of sulphuric acid and of a quantity of sodium aluminate corresponding to an Al/SiO₂ weight ratio of 0.20%). After this disintegration operation, the resulting slurry, with a pH equal to 8.4 and a loss on ignition equal to 79.0% (and therefore a solids content of 21.0% by weight), is sprayed by means of a nozzle sprayer.

The characteristics of the silica P2 obtained in the form of substantially spherical beads are as follows:

    ______________________________________                                         CTAB specific surface   135 m.sup.2 /g                                         BET specific surface    147 m.sup.2 /g                                         DOP oil uptake          250 ml/100 g                                           Zinc weight content     1.90%                                                  Pore volume V1 represented                                                                             0.86 cm.sup.3 /g                                       by the pores of d ≦ 400 Å                                           Pore volume V2 represented                                                                             0.39 cm.sup.3 /g                                       by the pores 175 Å ≦ d ≦ 275 Å                           V2/V1 ratio             45%                                                    pH                      8.5                                                    Mean particle size      220 μm                                              ______________________________________                                    

The number N of stearic acid molecules consumed per nm² of silica surface, when stearic acid is reacted with the said silica P2 in xylene for 2 hours at 120° C. (in accordance with the operating procedure outlined in the description), is equal to 1.7.

The silica P2 is subjected to the disintegration test as defined above in the description.

After disintegration with ultrasound it has a median diameter (.O slashed.₅₀ ) of 3.2 μm and an ultrasonic disintegration factor (F_(D)) of 14.5 ml.

The characteristics of the silicas prepared in Examples 1 and 2 and those of a commercial silica sold in the form of substantially spherical beads by RHONE-POULENC CHIMIE as a reinforcing filler for elastomers, in this case the silica ZEOSYL® 175 MP (referred to as MP1), are reported in Table 1 below.

                  TABLE I                                                          ______________________________________                                                      MP1         P1      P2                                            ______________________________________                                         S.sub.CTAB  (m.sup.2 /g)                                                                    162         145     135                                           S.sub.BET  (m.sup.2 /g)                                                                     175         175     147                                           DOP (ml/100 g)                                                                              280         275     250                                           Zn (%)       <0.005      1.80    1.90                                          V1 (cm.sup.3 /g)                                                                            0.95        0.95    0.86                                          V2 (cm.sup.3 /g)                                                                            0.45        0.40    0.39                                          V2/VI (%)    47          42      45                                            pH           6.5         8.5     8.5                                           Mean size    265         210     220                                           (μm)                                                                        N (mol/nm.sup.2)                                                                            0.5         1.4     1.7                                           .O slashed..sub.50  (μm)                                                                 9.1         2.7     3.2                                           F.sub.D  (ml)                                                                               2.1         16      14.5                                          ______________________________________                                    

EXAMPLE 3

This example illustrates the use and the behavior of a silica according to the invention and of a silica not in accordance with the invention in a formulation for industrial rubber.

The following formulation is employed (the parts are expressed by weight):

    ______________________________________                                         S.B.R. 1955 S 25 rubber .sup.(1)                                                                   50                                                         B.R. 1220 rubber .sup.(2)                                                                          25                                                         Natural rubber      25                                                         Silica              51                                                         Active ZnO .sup.(3) 1.8                                                        Stearic acid        0.35                                                       6PPD .sup.(4)       1.45                                                       CBS .sup.(5)        1.1                                                        DPG .sup.(6)        1.4                                                        Sulphur .sup.(7)    0.9                                                        Silane X50S .sup.(8)                                                                               8.15                                                       ______________________________________                                          .sup.(1)  Styrene butadiene copolymer solution type 1955 S 25                  .sup.(2)  Butadiene polymer type 1220                                          .sup.(3)  Rubber grade zinc oxide                                              .sup.(4)  N(1,3-Dimethylbutyl)-Nphenyl-p-phenylenediamine                      .sup.(5)  NCyclohexyl 2benzothiazyl sulphenamide                               .sup.(6)  Diphenyl guanidine                                                   .sup.(7)  Vulcanizing agent                                                    .sup.(8)  Silica/rubber coupling agent (product marketed by Degussa)     

The formulations are prepared in the following manner:

The following are introduced into an internal mixer (Banbury type), in this order and at the times and temperatures of the mixing which are shown in brackets:

S.B.R. 1955 S 25, B.R. 1220 and natural rubber (t₀)(60° C.)

the X50S and 2/3 of the silica (t₀ +1 min)(80° C.)

the ZnO, the stearic acid, the 6PPD and 1/3 of the silica (t₀ +2 min)(100° C.)

The discharge from the mixer (mix drop) takes place when the chamber temperature reaches 165° C., (that is to say at approximately t₀ +5 min 10 s). The mix is introduced onto a roll mill, the rolls being kept at 30° C., to be calendered thereon. The CBS, the DPG and the sulphur are introduced onto this mill.

After homogenization and three fine passes the final mixture is calendered into the form of sheets from 2.5 to 3 mm in thickness.

The results of the tests are the following:

1- Rheological properties

The measurements are carried out on the formulations in the raw state.

The results are reported in Table II below. The apparatus employed for conducting the measurements has been shown.

                  TABLE II                                                         ______________________________________                                                             MP1     P1                                                 ______________________________________                                         MOONEY consistency .sup.(1)                                                                        130     85                                                 Min. torque (ln.lb) .sup.(2)                                                                       26.5    19.1                                               ______________________________________                                          .sup.(1)  MOONEY MV 2000E viscometer (Mooney large (1 + 4) measurement at      100° C.)                                                                .sup.(2)  MONSANTO 100 S rheometer                                       

The formulation obtained from the silica according to the invention results in the lowest values.

This expresses a greater processibility of the mixes prepared from the silica according to the invention, in particular in respect of the extrusion and calendered operations which are often carried out during the manufacture of elastomer compositions (lower energy expenditure for processing the mix, greater ease of injection during the compounding, less die swell during extrusion, less shrinkage on calendering, . . . ). 2- Mechanical properties

The measurements are carried out on vulcanized formultaions.

The vulcanization is carried out by healing the formulations to 150° C. for 40 minutes.

The following standards were employed:

(i) tensile tests (moduli, tensile strength)

NFT 466002 or ISO 37-1977

(ii) tests of abrasion resistance

5 DIN 53-516

The results obtained are listed in Table III below.

                  TABLE III                                                        ______________________________________                                                              MP1    P1                                                 ______________________________________                                         300% modulus/100% modulus                                                                           3.4    5.4                                                Tensile strength (MPa)                                                                              17.1   20.8                                               Abrasion resistance (mm.sup.3) .sup.(1)                                                             58     54                                                 ______________________________________                                          .sup.(1)  the measured value is the loss on abrasion: the lower it is, th      better the abrasion resistance.                                          

These last results demonstrate the good reinforcing effect conferred by the silica according to the invention.

Thus, while resulting in more satisfactory rheological properties, the silica according to the invention provides mechanical properties which are better than those obtained with the silica of the prior art.

On the one hand, the silica according to the invention produces a 300% modulus/100% modulus ratio that is greater than the silica of the prior art, which is a proof of better dispersion of the silica within the rubber matrix.

On the other hand, the high reinforcing power of the silica according to the invention is confirmed by the high value obtained for the tensile strength.

At last, with regard to the abrasion resistance, it can be seen that the loss by abrasion is reduced in relation to the comparative silica. 3-Dynamic properties

The measurements are carried out on vulcanized formulations.

The vulcanization is obtained by heating the formulations to 150° C. for 40 minutes. The results (illustrating the tendency to heat up) are reported in Table IV below (the lower the value, the lower the tendency to heat up). The apparatus employed for conducting the measurements has been shown.

                  TABLE IV                                                         ______________________________________                                                              MP1    P1                                                 ______________________________________                                         Internal heating (°C.) .sup.(1)                                                              90     69                                                 ______________________________________                                          .sup.(1)  GOODRICH flexometer                                            

The tendency to heat up obtained from the silica according to the invention is low. 

What is claimed is:
 1. A process for preparing precipitated silica comprising reacting a silicate with an acidifying agent to produce a suspension of precipitated silica and, then, separating and drying the suspension, wherein the precipitation is carried out by the steps comprising:(i) forming an initial base stock comprising the silicate of alkali metal M and M₂ O, said stock initially having a silicate concentration (expressed as SiO₂ ) lower than about 20 g/l, (ii) adding the acidifying agent to said base stock until at least about 5% of the quantity of M₂ O present in said base stock is neutralized, and (iii) adding simultaneously additional acidifying agent and silicate to said base stock such that the ratio of the quantity of silicate added (expressed as SiO₂)/the quantity of silicate present in the initial base stock (expressed as SiO₂ ) is greater than about 4 and at most about 100, wherein said process further comprises one of the following two operations (a) or (b):(a) adding at least one zinc compound and then a basic agent to the base stock after stage (iii), said separation comprising a filtration to obtain a cake and disintegration of the cake, said disintegration being optionally performed in the presence of at least one aluminum compound, or (b) adding simultaneously additional silicate and at least one zinc compound to the base stock after stage (iii), and said separation comprising a filtration to obtain a cake and the disintegration of the cake, the disintegration being optionally performed in the presence of at least one aluminum compound.
 2. A process according to claim 1, wherein the precipitation is carried out by the steps comprising:(i) forming an initial base stock comprising a silicate of alkali metal M, said stock initially having a silicate concentration (expressed as SiO₂) lower than about 20 g/l, (ii) adding the acidifying agent to said base stock until at least about 5% of the quantity of M₂ O present in the said base stock is neutralized, (iii) adding simultaneously additional acidifying agent and silicate of alkali metal M to said base stock such that the ratio of the quantity of silicate, added (expressed as SiO₂ )/the quantity of silicate present in the initial base stock (expressed as SiO₂) is greater than about 4 and at most about 100, then (iv) adding one zinc compound to said base stock, (v) optionally adding a basic agent until a pH value of between about 7.4 and about 10 is obtained, (vi) optionally adding additional acidifying agent to the reaction base stock until a pH value of about 7 is obtained, andwherein the separation comprises a filtration to obtain a cake and disintegration of the cake, the disintegration being performed in the presence of at least one aluminum compound.
 3. A process for preparing precipitated silica comprising reacting a silicate with an acidifying agent to produce a suspension of precipitated silica and, then, separating and drying the suspension, wherein the precipitation is carried out by the steps comprising:(i) forming an initial base stock comprising the silicate of alkali metal M and M₂ O, said stock initially having a silicate concentration (expressed as SiO₂) lower than about 20 g/l, (ii) adding the acidifying agent to said base stock until at least about 5% of the quantity of M₂ O present in said base stock is neutralized, and (iii) adding simultaneously additional acidifying agent and silicate to said base stock such that the ratio of the quantity of silicate added (expressed as SiO₂ )/the quantity of silicate present in the initial base stock (expressed as SiO₂) is greater than about 4 and at most about 100, and (iv) adding simultaneously additional silicate and at least one zinc compound to the base stock.
 4. A process according to claim 2, wherein, between stage (iii) and stage (iv), acidifying agent is further added to said base stock, until a pH value of the reaction mixture of between about 3 and about 6.5 is obtained.
 5. A process according to claim 3, wherein, after stage (iv), acidifying agent is further added to said base stock.
 6. A process according to claim 3, wherein the separation comprises a filtration to obtain a cake and disintegration of the cake, the disintegration being preformed in the presence of a aluminum compound.
 7. A process according to claim 1, wherein, in stage (ii) the acidifying agent is added until at least about 50% of the quantity of M₂ O present in said initial base stock is neutralized.
 8. A process according to claim 1, wherein, in stage (iii), the quantity of silicate added (expressed as SiO₂)/quantity of silicate present in the initial base stock (expressed as SiO₂) is between about 12 and about
 100. 9. A process according to claim 1, wherein, in stage (iii), the quantity of silicate added (expressed as SiO₂)/quantity of silicate present in the initial base stock (expressed as SiO₂) is higher than about 4 and lower than about
 12. 10. A process according to claim 1, wherein, in stage (iii), said simultaneous addition of acidifying agent and of silicate is undertaken at a pH of between 7 and
 9. 11. A process according to claim 1, wherein no electrolyte is employed.
 12. A process according to claim 1, wherein the said silicate concentration expressed as SiO₂ in the said initial base stock is at most about 11 g/l.
 13. A process according to claim 1, wherein the said silicate concentration expressed as SiO₂ in the said initial base stock is at least about 8 g/l.
 14. A process according to claim 13, wherein the said silicate concentration expressed as SiO₂ in the said initial base stock is between about 10 and about 15 g/l.
 15. A process according to claim 1, wherein said filtration is performed by means of a filter press.
 16. A process according to claim l, wherein said drying is performed by spraying to obtain the precipitated silica as a dried product.
 17. A process according to claim 16, wherein said drying is performed with a multinozzle sprayer.
 18. A process according to claim 16, wherein the dried product is subsequently agglomerated.
 19. A process according to claim 16, wherein the dried product is further subsequently milled, then optionally agglomerated.
 20. A process according to claim 1, wherein the quantity of zinc compound which is employed is such that the precipitated silica prepared contains between about 1 and about 5% by weight of zinc.
 21. A process according to claim 1, wherein the zinc compound is an organic salt selected from the group consisting of salts of carboxylic acids and polycarboxylic acids.
 22. A process according to claim 1, wherein the zinc compound is an inorganic salt selected from the group consisting of halides, oxyhalides, nitrates, phosphates, sulphates and oxysulphates.
 23. A process according to claim 22, wherein the zinc compound is a zinc sulphate.
 24. A process according to claim 1, wherein the aluminum compound is an alkali metal aluminate.
 25. A process according to claims 24, wherein the aluminum compound is a sodium aluminate. 